This invention relates to an electrophotographic toner, and particularly to a toner having a distinguished transferability from a photosensitive material to a transfer sheet and a distinguished filming resistance to the photosensitive material.
Electrophotography is carried cut mainly by forming an electrostatic latent image on a photosensitive material such as selenium, zinc oxide or organic photoconductors, contacting the latent image with a mixture of carriers such as fine glass beads, iron powder, ferrite powder, etc., and electro-insulating, colored fine toners, the toners having been triboelectrically charged, thereby conducting an electrostatic development, then laying a transfer sheet on the developed image and transferring the developed image onto the transfer sheet under an electric field.
Heat-fixing type toners for use in the electrophotography contain a binder resin. Various thermoplastic and thermosetting resins are used as binder resins. The resins melt or soften by heating, and the image is fixed to the transfer sheet such as paper, etc. From the viewpoints of toner characteristics, such as electrostatic characteristics, flowability, etc. styrene resin, styrene-acrylonitrile copolymer resin, epoxy resin, polyester resin, etc. have been mainly used as the binder resin.
On the other hand, with recent higher printing speed in electrophotography, a better image quality has been required, and thus studies of an improvement in the image quality and the friability of an image have been initiated.
To improve the fixability of an image, the heating temperature for the fixation must be elevated so that the binder resin can be thoroughly melted. However, a small scale type or an energy-saving type has been recently required for copying machines or laser beam printers, and toners capable of melting and softening at a low temperature have been required without elevating the heating temperature.
A lower melting or softening temperature of toners means lowering of other characteristics of toners, such as toner flowability and toner preservation stability (toner coagulation resistance), and thus a better image quality is hard to obtain. Even if a better image quality is obtained, no better fixability of an image after the fixation can be obtained.
Furthermore, when the image-fixed sheet after the fixation is placed in a soft polyvinyl chloride bag, the toner image fixed on the sheet will be partially transferred to and deposited on the soft polyvinyl chloride surface, if styrene resin or styrene-acrylonitrile copolymer resin is used as a binder resin. Such a transfer seems to be caused by dissolution of the styrenic binder resin by a plasticizer in the soft polyvinyl chloride, for example, dibutyl phthalate or dioctyl phthalate.
Copied documents are often placed in transparent soft polyvinyl chloride bags and the transfer of toner images during the preservation as mentioned above has been a serious problem.
The transferability of the toner image to soft polyvinyl chloride hereinafter referred to as "PVC resistance") can be prevented by using epoxy resin as a binder resin. However, toner deposition onto a drum surface of selenium as a photosensitive material (toner filming) takes place earlier when the epoxy resin is used as a binder resin, and thus lowering of image contrast and fogging of white background and poor transfer of toner image are liable to take place. Thus, the life of photosensitive material itself will be apparently shorter and the maintenance of the photosensitive material will be more frequently required.
The heat fixation of electrophotography includes fixation by oven heating, flash fixation by a halogen lamp (U.S. Pat. No. 4,352,877), heat roll fixation with heat and pressure, etc., among which the heat roll fixation has a better fixability and can be made at a higher speed. Particularly with an increase in printing speed of copying machines, laser beam printers, etc. based on the electrophotographic process and also with a keener demand for high speed fixation, the heat roll fixation has been more and more employed. Heat roll fixation is carried out by pressing and heating with a heat roll made from a silicone rubber or fluoro-resin of good surface lubrication, further coated with a surface lubricant such as silicone oil, etc. or with a heat roll made from silicone rubber impregnated with silicone oil.
In the heat roll fixation, it is required that the so called offset phenomena, i.e. image fouling by deposition of a portion of toners onto the heat roll surface and further deposition onto paper because the toners in a heated and molten state are pressed onto the heat roll surface (hereinafter referred to as "offset resistance") may not take place. Needless to say, a fixation stability of image after the fixation is also required.
Styrene-grafted epoxy resin has been proposed as an intermediate resin between the styrenic resin and the epoxy resin [Japanese patent application Kokai (Laid-open) Nos. 58-203452 and 59-24865]. However, the resin, even though used as a binder resin, has an offset resistance no more than that of the single epoxy resin and rather has a poor PVC resistance because the styrene resin component is contained.